If your your list is dynamically generated with unknown number and your target is to always have last div in a new line set last div class to "col-xl-12" and remove other classes so it will always take a full row.
This is a copy of your code corrected so that last div always occupy a full row (I although removed unnecessary classes).
<link href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" rel="stylesheet">_x000D_
<div class="grid">_x000D_
<div class="row">_x000D_
<div class="col-sm-3">Under me should be a DIV</div>_x000D_
<div class="col-md-6 col-sm-5">Under me should be a DIV</div>_x000D_
<div class="col-xl-12">I am the last DIV and I always take a full row for my self!!</div>_x000D_
</div>_x000D_
</div>
_x000D_